1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and MRI method and, more particularly, to a magnetic resonance imaging apparatus and MRI method capable of discriminating the flow directions of fluids, such as bloodstreams, and suitable for magnetic resonance (MR) angiography.
2. Description of the Related Art
Recently, MR angiography for imaging only a bloodstream portion without using any contrast medium has attracted attention as non-invasive angiography. As a method of discriminating the flow directions of fluids in this angiography, a pre-saturation method is known.
The principle of this pre-saturation method will be described with reference to FIGS. 1A to 1C. Consider the case wherein, as shown in FIG. 1A, two blood vessels BD1 and BD2 extend through three parallel slices A, B, and C substantially perpendicularly to these slices.
First, as shown in FIG. 1B, a nucleus spin in the slice A is excited sufficiently to saturation beforehand by a 90.degree. pulse. Consequently, the direction of the nuclear spin in the slice A is rotated 90.degree. from the z'-axis to x'-axis direction in a rotating coordinate system. Thereafter, as shown in FIG. 1C, the slice B is similarly excited by the 90.degree. pulse, and an MR signal from the slice B is acquired. Since a bloodstream flowing into the slice B through the blood vessel BD1 is already saturated in the slice A, the signal value of the blood vessel BD1 is so small that the signal forms a blank image in black in an MR image of the slice B. On the contrary, a bloodstream flowing into the slice B through the blood vessel BD2 in a direction opposite to that of the blood vessel BD1 appears in the MR image of the slice B.
Subsequently, although not shown, a nucleus spin in the slice C is excited sufficiently to saturation by the 90.degree. pulse in the same manner as described above. Thereafter, the slice B is similarly excited by the 90.degree. pulse, and an MR signal from the slice B is acquired. Since, in this case, the bloodstream flowing into the slice B through the blood vessel BD2 is already saturated in the slice C, the signal value of the acquired MR signal is small, so that the signal of the blood vessel BD2 forms a black blank image in the MR image of the slice B. However, the bloodstream flowing into the slice B through the blood vessel BD1 appears in the MR image of the slice B.
Hence, the flow directions of the blood vessels BD1 and BD2 can be discriminated by checking which of the slices A and C adjacent to the signal acquisition slice B is pre-saturated when the blank image is formed. Note that the excitation pulse for the slice B is not limited to a 90.degree. pulse but may be a 180.degree. pulse.
To discriminate bloodstream flow directions by the above pre-saturation method, however, it is necessary to perform the signal acquisition and image reconstruction twice in the two directions. This results in a long imaging time and a reduction in throughput of patients. In addition, when the imaging time is prolonged, there is a high possibility that a patient to be examined or a portion to be examined moves, thus interfering acquisition of a correct diagnostic image.